Morphology and trap luminescence in thin oligothiophene films on HOPG

The correlation of photoluminescence (PL) and morphological properties of thin quater – (4T) and sexithiophene (6T) films vapour-deposited on highly oriented pyrolytic graphite (HOPG) was studied for various growth conditions. Atomic force microscopy and optical microscopy data reveal that the films are not continuous but consist of two different types of very large (~10 m) crystallites with varying contributions. One type has a compact irregular shape, the other one is more needle-like. PL-spectra of both show a highly resolved vibronic finestructure, indicating a low concentration of structural defects in the films. For thicker 6T films, an additional emission from deep trap states (580 cm?1 below the 0–0 transition) is observed. The correlation of the morphological and the optical data suggests that this trap luminescence is due to structural defects like grain boundaries of the compact crystallites.